Pastoral-based animal production systems are under increasing pressure to provide the high quantity and quality of feed needed for optimal ruminant performance. The capacity of farmers to increase forage yield further, solely by increasing fertilizer inputs or through improved pasture management, is limited. Emerging requirements to balance industry production targets against the need to reduce greenhouse gas emissions and N losses pose further challenges. Plant breeding is being asked to deliver results more urgently than at any time previously, and this review attempts to highlight issues that might limit the prospects for future progress by seeking lessons from four past examples: (i) white clover breeding gains and the need to consider the complexity of the grazed grass-clover mixed sward, with its tendency for cycling in plant species composition; (ii) a systems field trial of new and old grass ⁄ clover cultivars, and how the complexity of growth of perennial forage crops, and the dynamic optimality required for sustainable harvesting might limit our ability to breed for 'yield' per se; (iii) the manipulation of a physiological trait (low 'maintenance' respiration) and the implications of such changes for plant fitness and G · E interactions; and (iv) an hypothesis-driven development of a trait (high-sugar grasses) and the value of 'proof of concept' studies, the requirement of scientific understanding of the mechanisms of trait expression, and how one might in future go about assessing breeding achievements. We discuss the general ecological considerations around shifts in the frequency distribution of traits in new populations, whether altered conventionally or by genetic modification, and how selection for a particular trait might inadvertently reduce both fitness and persistence. A major priority for breeding, we propose, might be to revisit previously abandoned traits that affected the physiological performance of forage species, armed now with a capacity to monitor gene expression at the molecular level, and so unravel ⁄ control the G · E interactions that limited their benefits. We also discuss how a 'loss of yield advantage' of new cultivars, seen when tested several years after sowing, requires urgent investigation and propose this might be associated with fitness costs of perenniality. Finally, we argue for a careful reconsideration of what are realistic expectations for systems field trials and that focus on forage breeding might be shifted more to 'proof of concept' studies, critical experimental design, comparing 'traits' rather than 'cultivars', and the wider ecological assessment of fitness and function of traits in the plant, community and ecosystem.

The root morphology of ten temperate pasture species (four annual grasses, four perennial grasses and two annual dicots) was compared and their responses to P and N deficiency were characterised. Root morphologies differed markedly; some species had relatively fine and extensive root systems (Vulpia spp., Holcus lanatus L. and Lolium rigidum Gaudin), whilst others had relatively thick and small root systems (Trifolium subterraneum L. and Phalaris aquatica L.). Most species increased the proportion of dry matter allocated to the root system at low P and N, compared with that at optimal nutrient supply. Most species also decreased root diameter and increased specific root length in response to P deficiency. Only some of the species responded to N deficiency in this way. Root morphology was important for the acquisition of P, a nutrient for which supply to the plant depends on root exploration of soil and on diffusion to the root surface. Species with fine, extensive root systems had low external P requirements for maximum growth and those with thick, small root systems generally had high external P requirements. These intrinsic root characteristics were more important determinants of P requirement than changes in root morphology in response to P deficiency. Species with different N requirements could not be distinguished clearly by their root morphological attributes or their response to N deficiency, presumably because mass flow is relatively more important for N supply to roots in soil.

Control over the quantity and quality of food ingested by grazing ruminants in temperate pasture systems remains elusive. This is due in part to the foraging choices that animals make when grazing from communities of mixed plant species. Grazing behavior and intake interact strongly with the feed supply–demand balance, pasture composition, and grazing method. These interactions are not completely understood, even for relatively simple pasture communities such as a perennial ryegrass (Lolium perenne L.)–white clover (Trifolium repens L.) mixture. When offered a free choice between these species, ruminants exhibit a partial preference for clover compared to grass (about 0.7:0.3) and have a higher intake rate from clover but do not graze to maximize their daily intake of dry matter (DM). When monocultures of grass and clover are offered as a free choice in 50:50 area ratio, animal performance is no different than from a clover monoculture alone. Thus, all of the feeding value benefits of clover are available when only 0.5 of the grazing area is sown to clover. These observations accord with the satiety theory and imply that there are constraints to eating pure clover that animals can overcome by adding grass to their diet, provided their ability to locate and ingest each food is not seriously limited. The challenge for grassland management is to present feed to animals at pasture in ways that allow them to meet their dietary preferences, while also allowing high rates of animal production per hectare.

DairyMod and EcoMod, which are biophysical pasture-simulation models for Australian and New Zealand grazing systems, are described. Each model has a common underlying biophysical structure, with the main differences being in their available management options. The third model in this group is the SGS Pasture Model, which has been previously described, and these models are referred to collectively as ‘the model’. The model includes modules for pasture growth and utilisation by grazing animals, water and nutrient dynamics, animal physiology and production and a range of options for pasture management, irrigation and fertiliser application. Up to 100 independent paddocks can be defined to represent spatial variation within a notional farm. Paddocks can have different soil types, nutrient status, pasture species, fertiliser and irrigation management, but are subject to the same weather. Management options include commonly used rotational grazing management strategies and continuous grazing with fixed or variable stock numbers. A cutting regime simulates calculation of seasonal pasture growth rates. The focus of the present paper is on recent developments to the management routines and nutrient dynamics, including organic matter, inorganic nutrients, leaching and gaseous nitrogen losses, and greenhouse gases. Some model applications are presented and the role of the model in research projects is discussed.